Android实现美团外卖底部导航栏动画

private void initAnim() {
  value = startValue;
  animator = ValueAnimator.ofInt(startValue, halfValue, endValue);
//  animator.setInterpolator(new DecelerateInterpolator());
  animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
   @Override
   public void onAnimationUpdate(ValueAnimator valueAnimator) {
    value = (int) valueAnimator.getAnimatedValue();
    invalidate();
   }
  });
  animator.setDuration(ANIM_TIME);
  animator.start();
 }

startValue = 0;
halfValue = -DensityUtils.dp2px(context, 35);
endValue = -DensityUtils.dp2px(context, 25);

rectF = new RectF((((float)width - (float)contentSize) / 2),
    lineToTop + value,
    (float) width - ((float)width - (float)contentSize) / 2,
    height + value);
 
 
  if (value < 0 ) {
   Path path = new Path();
   //绘制第一段圆弧
   path.moveTo(rectF.left - 20, lineToTop);
   float firstCubicHight = ((float) lineToTop - rectF.top) / 4;
   float end = (float) lineToTop - firstCubicHight;
   path.cubicTo(
     rectF.left - 10,
     (float) lineToTop - firstCubicHight / 16,
     rectF.left,
     (float) lineToTop - firstCubicHight / 12,
     rectF.left + 10,
     end
   );
 
   //绘制第二段圆弧
 
   path.quadTo(
     rectF.left + contentSize / 2,
     rectF.top * 0.9f,
     rectF.right - 10,
     end
   );
   //绘制第三段圆弧，和第一段对称
 
   path.cubicTo(
     rectF.right,
     (float) lineToTop - firstCubicHight / 12,
     rectF.right + 10,
     (float) lineToTop - firstCubicHight / 16,
     rectF.right + 20,
     lineToTop
   );
 
   Path pathBg = new Path();
   pathBg.addPath(path);
 
   pathBg.lineTo(rectF.right + 20, height);
   pathBg.lineTo(rectF.left - 20, height);
   pathBg.lineTo(rectF.left - 20, lineToTop);
   canvas.drawPath(pathBg, paintBg);//绘制白色背景
 
   canvas.drawPath(path, paintBgStroke);//绘制曲线
 
  }